Dynamic zonation of Nerita plicata, N. undata and N. textilis (Prosobranchia: Neritacea) populations on a rocky shore in Kenya.

Abstract

Vertical zonation of Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, inhabiting the rocky cliffs of Nyali, Kenya, was studied through several spring-neap tidal cycles from November to December (northeastern monsoon) 1981 and May to July (southeastern monsoon) 1982. For the first time, a monsoon-related cycle of migration in Nerita plicata and N. undata , superimposed on the shorter term spring-neap cycle is demonstrated. Both species rest predominantly in the upper eulittoral zone and the littoral fringe. N. plicata and N. undata rest at significantly higher levels during the southeastern monsoon (SEM) than during the northeastern monsoon (NEM) (P < 0.001). On and around neap-tide days N. plicata, N. undata , and N. textilis occupy significantly distinct mean resting positions (P < 0.001) in both season; N. undata being the highest in the SEM, followed by N. plicata , with N. textilis always in the lowest level.